1. Field of the Invention
The present invention relates to a wire dot printer head and a wire dot printer.
2. Discussion of the Background
A wire dot printer head is an apparatus which performs printing by rocking, between a waiting position and a printing position, an armature with a printing wire connected thereto, to strike the front end of the wire on a printing medium such as a sheet when the armature is rocked to the printing position.
As the wire dot printer head, there has been developed an apparatus for printing by the use of a magnetic circuit which attracts an armature from a waiting position to a printing position with a magnetic flux generated by a coil around the armature to be rocked.
In such a wire dot printer head, the armature has an arm which supports a wire for printing. The wire for printing is supported in such a manner that the armature is rotatable on the center of a support shaft in a direction in which the armature moves away from a core wound with a coil. The armature is being pressed by a pressing member such as a coil spring in a direction in which it moves away from the core. (Refer to Japanese Patent Laid-Open No. Hei 10-291330 and Japanese Patent No. 2833001.) In this case, the pressing member is located in a position in which it comes in contact with the arm of the armature. Thus, the pressing member is in constant contact with the arm of the armature.
With a recent trend toward faster printing operation, however, the armature rocks between the printing position and the waiting position at such a high speed as 2500 times per second. Therefore, the contact area of the arm which is in contact with the pressing member is subject to gradual abrasion by the pressing member, with the result that the armature will finally be broken at the contact area. Furthermore, to realize the further speedups of printing operation, armature weight reduction is demanded. The armature arm is formed as thin as possible, about 0.20 mm for example, for the purpose of decreasing the moment of inertia caused by the rocking of the armature. The armature arm, therefore, is an easy-to-break structure. Here, in the case that the arm is formed of, for example, a heat-treated 0.20 mm-thick SK-5 plate, the contact area of the arm which is in contact with the pressing member has as low durability as only about 30 k dots. With this durability, the armature is liable to failures at an early time, and consequently a shortened life of the wire dot printer head will result.